Mixtures of branched and linear

Several assumptions were made in using the broad MWD standard approach for calibration. With some justification a two parameter equation was used for calibration however the method did not correct or necessarily account for peak speading and viscosity effects. Also, a uniform chain structure was assumed whereas in reality the polymer may be a mixture of branched and linear chains. To accurately evaluate the MWD the polymer chain structure should be defined and hydrolysis effects must be totally eliminated. Work is currently underway in our laboratory to fractionate a low conversion polydlchlorophosphazene to obtain linear polymer standards. The standards will be used in polymer solution and structure studies and for SEC calibration. Finally, the universal calibration theory will be tested and then applied to estimate the extent of branching in other polydlchlorophosphazenes. 

Hydridotris(pyrazolyl)borate (N,N,N)-containing ruthenium(ll) complexes activate benzene in stoichiometric amounts to give the isolable complex B. A catalytic hydroarylation of ethylene led to ethylbenzene and a ca. 1 1 mixture of branched and linear alkylbenzenes was obtained when employing propylene (Equation (62)).63,63a... 

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

Ni(PMe2Ph)4, as exemplified in Table 8. However, the use of dimethyl phos-phonate usually results in the formation of a mixture of branched and linear isomers. Addition of diphenylphosphinic acid to the reaction system significantly improves the selectivity for the branched isomer (see below). 

Reformatsky reaction.5 The Reformatsky reaction of y-bromocrotonic acid with aldehydes or ketones in THF gives rise to a mixture of branched and linear unsaturated hydroxy acids. The former are the kinetic products and, on equilibration, rearrange to the latter products. 

Since the twenties and thirties butanol together with acetone was produced by fermentation of carbohydrates (corn). In the sixties the process was replaced by the hydroformylation of propene. In the OXO process alkenes react with synthesis gas in the presence of a homogeneous catalyst to give a mixture of branched and linear aldehydes ... 

Doye s group [81] showed that a dinuclear titanium-sulfonamidate complex (Scheme 21), with a tetrahedral sulfur in the ligand backbone, can be used for intermo-lecular hydroaminoalkylation as well. This system gives mixtures of branched and linear products, although to date there has been no mechanistic rationale provided for the reduced regioselectivity of group 4 metal complexes in this transformation. There has been one report by Zi s group [44] that describes axially chiral bis(sulfonamidate) tantalum and niobium complexes for application as precatalysts for hydroamination and hydroaminoalkylation. Unfortunately, these complexes did not show any reactivity for either of these reactions. 

Analogous allylic oxidations of higher olefins over heterogeneous palladium catalysts afford mixtures of products owing to the unsymmetrical nature of the f-allyl intermediate. For example, cis- or trans-2-butene gives a mixture of branched and linear allylic acetates. With olefins containing more than four carbon atoms palladium-catalyzed isomerization can lead to complex mixtures of products, which severely restricts the synthetic scope of these reactions. 

A solder resistant high-temperature composition that does not suffer from this drawback has been developed. The blend is composed of poly-(arylene ether) (PAE), PPS, and GFs. The PAE has an intrinsic viscosity (IV) less than or equal to about 0.15 dlg as determined in chloroform at 25°C. The use of the low IV PAE permits improved blending, which leads to improved high-temperature properties. Homopolymers of PAE are those containing 2,6-dimethylphenylene ether units. Suitable copolymers include random copolymers containing, for example, 2,6-dimethylphenylene ether units. In combination with 2,3,6-trimethyl-l,4-phenylene ether units or alternatively, copolymers derived from the copolymerization of 2,6-dimethylphenol with 2,3,6-trimethylphenol. Partially crosslinked PPS, as well as mixtures of branched and linear PPS, may be used in the high-temperature compositions. 

Regioselectivity is one of the major problems of Mizoroki-Heck reactions. It is supposed to be affected by the type of mechanism ionic versus neutral, when the palladium is ligated by bidentate P P ligands. The ligand dppp has been taken as a model for the investigation of the regioselectivity. Cabri and Candiani [Ig] have reported that a mixture of branched and linear products is formed in Pd°(P P)-catalysed Mizoroki-Heck reactions performed from electron-rich alkenes and aryl halides (Scheme 1.26a) or aryl ttiflates in the presence of halide ions (Scheme 1.26b). This was rationalized by the so-called neutral mechanism (Scheme 1.27). The neutral complex ArPdX(P P) is formed in the oxidative addition of Pd°(pAp) yj Qj. Q aj.yj triflates in the presence of halides. The carbopalladation... 

In more recent studies by Xiao and coworkers [40m,n], Mizoroki-Heck reactions catalysed by Pd(OAc)2 associated with dppp and performed from the eleclron-rich alkene ( -butylvinyl ether) and aryl halides (without any halide scavenger, i.e. under the conditions of the textbook neutral mechanism of Scheme 1.27 proposed by Cabri and Candiani [Ig]) give a mixture of branched and linear products in DMF, whereas the branched product is exclusively produced in ionic liquids (in the absence of halide scavengers) in a faster reaction. Whatever the medium, the cationic complex ArPd5(dppp)+ is always the sole reactive complex with electron-rich alkene (Scheme 1.33) [53]. Consequently, the regioselectivity should not vary with the experimental conditions. 

In the catalytic reactions of Xiao and coworkers [40m,n] performed in DMF from aryl bromides and -butylvinyl ether, a mixture of branched and linear products is formed... 

The regular windows and channels in zeolites with specific dimensions are the basis for their shape-selective properties and accordingly their utiUzation in purification of gas mixtures or mixtures of branched and linear hydrocarbons. The ability to preferentially adsorb certain molecules and simultaneously excluding others led to the introduction of the term molecular sieves for zeoUtes and their appUca-tions in separation techniques based on their size-exclusion properties. 

Fig. 29 Phase diagram of the CH/AC/PI system at 25°C. The polymer sample PI (M = 5 kg/mol, Mw = 12 kg/mol) consists of a mixture of branched and linear chains. Crossed circles cloud points, half-closed circles overall composition of the coexistence experiments, open circles compositions of the polymer-lean phases, closed circles compositions of the polymer-rich phases, closed square swelling point of PI in AC. The composition area of possible demixing is hatched [71]...

A successful theory, or simulation method, for branched polymer rheology should predict the frequency-dependent linear viscoelasticity of an arbitrary mixture of branched (and linear) spedes, whether those species are stars, H molecules, combs, or irregular branched structures. Such a theory, if it could be developed, would allow one to design polymer branching structures to produce the desired linear viscoelastic response. It could also potentially be used to infer from linear viscoelastic data information about the type of branching present in the melt [56, 57,5]. This latter use of linear viscoelastic data is referred to as analytical rheology. 

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